The present invention relates to resistive metal foils, and more particularly, to metal foils such as copper foils which are used in the production of a variety of products including printed circuit boards, and which include resistive metals embedded in the boards. More particularly, the invention relates to improving adhesion between the resistive metal of resistive metal foils and insulating laminating materials such as prepregs.
Printed circuit boards are currently used as the substrate materials in a wide variety of electronic devices. Typically, these boards are fabricated from a thin sheet of copper foil laminated to either a fiberglass/epoxy hard board or, in some instances, flexible plastic substrates. During the latter stages of the fabrication, the copper foil is printed with the necessary circuit pattern, and the unnecessary portions of the copper foil are then etched away to provide the desired interconnecting circuitry between various components in the electronic circuit design.
Resistive metal foils are metal foils including one or more resistive areas, in which a resistor is formed as an integral part of the metal foil. Embedding resistors in a printed circuit or wiring board both reduces the size of the package and improves reliability and electrical performance. Integrating resistors into the printed circuit board laminate substrate frees up the surface area of the board which would otherwise be taken by the resistor, enabling increased device functionality by placement of a greater number of active components on the surface of the laminate. Resistive foils may be made by vacuum deposition of a thin film resistive material, such as a resistive nickel-chromium alloy onto a conductive metal foil, such as a copper or copper-alloy based foil, through a roll-to-roll process. To enhance uniform resistivity of the resistive foil, the thin film resistive material layer may be directly deposited on the matte or shiny side of the conductive metal foil. In many applications, mechanical adhesion improving treatment, such as nodular or dendritic metal deposition, is not used prior to deposition of the resistive material layer on the conductive metal foil. Subsequent to the application of the resistive material layer, the resistive foil may be applied to an insulating laminating material, such as a prepreg, to form a laminate containing the resistive foil. In some applications, when the resistive material contains a material known as an adhesion promoting material, such as chromium, the peel strength of the resistive foil to the insulating laminating material is adequate without the addition of a further adhesion-promoting material.
The operational speed and signal integrity of an electronic device is influenced by the dielectric material which encapsulates the conductive copper line in a printed circuit board laminate. Laminate materials are changing to enable higher speed and better signal integrity by reducing dielectric constant and loss tangent by removing from the resin matrix hydroxyl and other polar groups responsible for signal degradation. The polar groups contribute to good bond strength between copper and resin. The absence of the polar groups at the copper/resin interface leads to low adhesion.
However, in some applications, the peel strength is inadequate to maintain adhesion between the resistive metal layer and the insulating laminating material used in the laminate. Accordingly, what is needed is an adhesion promoting material for use in such applications.
This invention relates to a process to enhance adhesion of resistive foil to laminating materials, including providing a metal foil; depositing at least one resistive metal layer on a side of the metal foil; applying at least one layer of at least one adhesion-promoting material over and adhered to the resistive metal layer; and laminating the resistive metal layer to a laminating material, the adhesion-promoting material providing enhanced adhesion between the resistive metal layer and the laminating material having a peel strength of at least 2.5 lb/in. The invention further relates to a metal body including a metal foil layer; a resistive metal layer overlying and adhered to the metal foil layer; and an adhesion-promoting material layer overlying and adhered to the resistive metal layer, the adhesion-promoting material providing enhanced adhesion between the resistive metal layer and a laminating material having a peel strength of at least 2.5 lb/in. The invention further relates to laminates including a metal foil layer; a resistive metal layer overlying and adhered to the metal foil layer, an adhesion-promoting material layer overlying and adhered to the resistive metal layer; and a layer of an electrically non-conductive material overlying and adhered to the adhesion-promoting layer, the adhesion-promoting material providing enhanced adhesion between the resistive metal layer and the laminating material having a peel strength of at least 2.5 lb/in. In one embodiment, the resistive metal layer comprises NiCr, NiCrAlSi, aluminum, nickel, zinc, titanium, vanadium, chromium, manganese, iron, tantalum, molybdenum, ruthenium, and alloys, oxides, nitrides and silicides thereof.
Thus, the present invention relates to a metal foil having a resistive metal layer and improved adhesion between the resistive metal layer and laminating materials which may be applied thereto. The present invention provides a solution to the problem of poor adhesion between resistive metals and certain laminating materials used in making, e.g., PCBs.